In a wireless communication system, a high power amplifier (HPA) is used to boost the signal power to the level necessary for transmission over the required communication range. To achieve the maximum power efficiency, the HPA is required to operate at or near the saturation point. However, an HPA cannot deliver an increased output power beyond the saturation point even with increased input signal level. Thus, the input signal average power should produce an output power at or near the saturation point of the HPA. If the input signal has amplitude variation with high peak-to-average power ratio (PAPR), the transmitted power of the HPA must be backed off from the saturation point by an amount proportional to the PAPR, thus reducing power efficiency. Otherwise, the non-linearity caused by the saturation will introduce inter-modulation components in the signal frequency band, cause signal distortion, and hence degrade system performance. In a digital communication system, a signal with large PAPR also has a large dynamic range, which requires the use of high quality analog-to-digital and digital-to-analog converters, resulting in increased complexity and cost. Therefore, reducing the PAPR of the input signal to the HPA is an important way to improve power efficiency, increase communication range, reduce power consumption, and reduce cost and complexity for a wireless communication system.
Orthogonal frequency-division multiplexing (OFDM) and code-division multiple access (CDMA) are two widely used wireless communication techniques. OFDM and its multi-user version, orthogonal frequency-division multiple access (OFDMA), offer high spectral efficiency, robustness against multipath propagation and channel fading, and low implementation complexity. However, an OFDM/OFDMA signal typically exhibits a large PAPR, which is one of the primary disadvantages of such systems. CDMA allows multiple users to share a communication channel with each user's data symbols “spread” by a spreading code. Signals intended for different users using their respective spreading codes are combined together at the base station for downlink transmission. As the number of users increases, the combined signal also exhibits a large PAPR.
A number of PAPR reduction techniques have been proposed in the literature. These techniques include clipping, coding, phase optimization, nonlinear companding, tone reservation and tone injection, constellation shaping, partial transmission sequence and selective mapping. Among these techniques, clipping and its variant, peak windowing, are the most straightforward ways to reduce the PAPR of a signal. These techniques are widely applied in practical systems due to the following advantages. First, they do not require any side information about date modulation or any redundant dummy code or subcarrier to be transmitted, and hence there is no loss of data rate or spectral efficiency after PAPR reduction. Second, they do not require iterative computation, and hence have lower complexity than the other techniques. Third, they do not require any modification of the receiver structure, and can be applied to any signal waveform in any wireless communication system, including OFDM/OFDMA and CDMA systems.
However, the clipping technique introduces both in-band distortion and out-of-band radiation. Though filtering after clipping can reduce the out-of-band radiation, it can also cause peak re-growth after filtering and increase the complexity. The peak windowing variant produces much less out-of-band radiation, but may cause over-attenuation or under-attenuation of the input signal.